Giant Asymmetric Second-Harmonic Generation in Bianisotropic Metasurfaces Based on Bound States in the Continuum

Bianisotropy is a powerful concept enabling asymmetric optical response, including asymmetric reflection, absorption, optical forces, light trapping, and lasing. The physical origin of these asymmetric effects can be understood from magnetoelectric coupling and asymmetrical field enhancement. Here, we theoretically propose highly asymmetric second-harmonic generation in bianisotropic AlGaAs metasurfaces. We show that the ratio of second-harmonic power for the forward and backward illuminations can be increased by around four orders of magnitude. This effect is obtained by altering geometrical parameters that coincide with quasi-bound states in the continuum. Our study paves the way toward a directional generation of higher-order waves and can be potentially useful for nonlinear holograms.